The 2006 Lexmark Indy300 led to the construction of a small amplifier to provide us with music throughout the week. The expectation was that it will get absolutely ruined by drunks so it needed to be cheap and fast to build. Enter the Gainclone.
Gainclone amplifiers started on in the DIY communities as a rip-off of the 47 Laboratories GainCard amplifier. The amplifier is said to sound quite good so it was surprising when a teardown revealed a simple National Semiconductor Overture audio amplifier on a chip inside.
This Gainclone is built around the LM4766 now owned by Texas Instruments. It is a dual 40watt power amplifier on a chip which can be configured in a bridged configuration. When bridged each half of the amplifier will drive an opposing signal to the speaker resulting in double the power output, however each channel sees only half the impedance of the speaker making this configuration most suitable to high impedance (8Ω) loads.
The amplifier input is taken from an input selector switch and then volume is controlled via a simple potentiometer. The powersupply is is a straight 300VA torroid with +/- 35V secondaries, rectified and then smoothed by 4x 10000μF capacitors per rail.
The amplifier worked as intended and is currently in service at home with a spare set of speakers. Total build cost was less than $100.